Poly-α-olefins (PAOs) are frequently used in industry as synthetic lubricant base oils for lubricating oils such as automobile gear oils, engine oils, industrial lubricating oils and hydraulic oils. Such PAOs may be obtained by oligomerizing higher α-olefins with acid catalysts (see, for example, Patent Literatures 1 to 3). Meanwhile, it is known that ethylene⋅α-olefin copolymers are also usable as synthetic lubricating oils excellent in viscosity index, oxidation stability, shear stability and heat resistance (see, for example, Patent Literature 4).
In recent years, the conditions under which lubricating oils are used have become more extreme and at the same time the extension of life is required out of consideration for environmental problems. Thus, while there has been a tendency toward an increase in demands for synthetic lubricating oils such as PAOs and ethylene⋅propylene copolymers having excellent low-temperature viscosity characteristics, heat resistance and oxidation stability, further improvements in viscosity index and low-temperature viscosity characteristics are desired from the points of view of fuel efficiency and energy saving.
In particular, the request for higher fuel efficiency has been increasing day by day and consequently an approach is adopted in which the viscosity of lubricating oils themselves is lowered in order to reduce the resistance incurred during stirring of the lubricating oils. This approach also gives rise to a risk that metallic parts are brought into contact with each other due to the depression in lubricating performance. Thus, the designing of lubricating oils involves considering the balance between the conditions and loads under which the oils are used, and the viscosity of the lubricating oils. Such designing should take into account the drop in viscosity ascribed to the degradation of lubricating oils during use, specifically, the breakage of the molecules of lubricant oil materials mainly due to shear stress. Such a decrease in viscosity increases the risk of metallic contact between gears or bearings. It is therefore usually necessary that the initial viscosity of lubricating oils as produced be higher than the designed optimum viscosity by a degree depending on the shear stability of the materials used. Thus, there has been a strong need for lubricating oils with excellent shear stability in order to attain the maximum reduction in initial viscosity.
The production of ethylene⋅α-olefin copolymers used as synthetic lubricating oils conventionally involves a vanadium catalyst including a vanadium compound and an organoaluminum compound (see, for example, Patent Literatures 5 and 6). The main ethylene⋅α-olefin copolymers produced by such a method are ethylene⋅propylene copolymers.
Further, catalyst systems including a metallocene compound such as zirconocene and an organoaluminum oxy compound (aluminoxane) are known to afford copolymers with high polymerization activity (see, for example, Patent Literatures 7 and 8). Patent Literature 9 discloses a method for producing a synthetic lubricating oil that includes an ethylene⋅α-olefin copolymer obtained with use of a catalyst system combining a specific metallocene catalyst and an aluminoxane.
Ethylene⋅α-olefin copolymers used in lubricating oils desirably have high randomness in order to meet low-temperature viscosity characteristics that are required. In solution polymerization, high polymerization temperatures are generally considered as preferable because the productivity is enhanced. However, it is known to those skilled in the art that the randomness of olefin polymers produced is decreased with increasing polymerization temperature. Because of this fact, the upper limit of the polymerization temperature is depressed in many cases. Thus, polymerization catalysts which can give highly random olefin polymers even at high polymerization temperatures are desired in order to solve such problems.
Further, ethylene⋅α-olefin copolymers used in lubricating oils are required to be stable to oxidation. It is therefore desirable that the copolymers have a small number of double bonds. Although the number of double bonds can be reduced by hydrogenating the copolymers, copolymerization directly giving polymers with less double bonds is advantageous in that the process is simplified. Thus, polymerization catalysts capable of affording copolymers having less double bonds are desired.